Half-moon lifting device

ABSTRACT

A pipe coil lifting device includes a superstructure with a central longitudinal member and at least two supports attached to opposite sides of the central longitudinal member, with two channels at the outside long-edges of the superstructure attached to the supports, with a convex upper surface attached to the supports of the superstructure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/235,906 which is a continuation of U.S. patent application Ser. No.16/154,338 which is a continuation-in-part of U.S. patent applicationSer. No. 15/941,606, entitled “HALF-MOON LIFTING DEVICE” filed Mar. 30,2018, which is herein incorporated by reference in its entirety for allpurposes.

BACKGROUND

Flexible pipe is useful in a myriad of environments, including in theoil and gas industry. Flexible pipe may be durable and operational inharsh operating conditions and can accommodate high pressures andtemperatures. Flexible pipe may be bundled and arranged into one or morecoils to facilitate transporting and using the pipe.

The flexible pipe may be transported as coils to various sites fordeployment (also referred to as uncoiling or unspooling). Differenttypes of devices and vehicles are currently used for loading andtransporting coils of pipe, but usually extra equipment and human manuallabor is also involved in the process of loading or unloading such coilsfor transportation and/or deployment. Such coils of pipe are often quitelarge and heavy. Accordingly, there exists a need for an improved methodand apparatus for loading and unloading coils of pipe.

SUMMARY

Various nonlimiting embodiments provide methods and apparatus for movingcoils of flexible pipe. In one nonlimited embodiment, a pipe coillifting device includes a superstructure with a central longitudinalmember and at least two supports attached to opposite sides of thecentral longitudinal member, with two channels at the outside long-edgesof the superstructure attached to the supports, with a convex uppersurface attached to the supports of the superstructure.

In other nonlimiting embodiments, a pipe coil lifting device includes asuperstructure having a pair of longitudinal channel portions attachedto and forming sides of an outermost portion of the superstructure, alongitudinal central member extending between the channel portionshaving at least two supports perpendicular and attached to oppositesides of the longitudinal central member and channel portions, andwherein the top of the superstructure forms a convex upper surfaceattached to the channel portions, central member and supports.

In further nonlimiting embodiments, method for lifting a pipe coilincludes lifting a coil of flexible pipe with a pipe coil liftingdevice, the pipe coil lifting device comprising a superstructure with acentral member and at least two supports perpendicular and attached toopposite sides of the central member, with two channels at the outsidelong-edges of the superstructure, with a convex surface attached to thesupports of the superstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredin conjunction with the following drawings, in which:

FIG. 1 is a diagram of a pipe coil lifting device mounted on tines of aforklift according to embodiments of the present disclosure;

FIG. 2 is a diagram of a coil of pipe according to embodiments of thepresent disclosure;

FIG. 3 is a diagram of a coil of pipe lifted disposed on a pipe coillifting device using a forklift according to embodiments of the presentdisclosure;

FIG. 4 illustrates a top view of a pipe coil lifting device according toembodiments of the present disclosure;

FIG. 5 illustrates a view from under a pipe coil lifting deviceaccording to embodiments of the present disclosure;

FIG. 6 illustrates a cross-sectional view a pipe coil lifting deviceaccording to embodiments of the present disclosure;

FIG. 7 illustrates a cross-sectional view of a pipe coil lifting deviceaccording to embodiments of the present disclosure;

FIG. 8 illustrates a cross-sectional view of a pipe coil lifting deviceaccording to embodiments of the present disclosure;

FIG. 9 illustrates an end member for a pipe coil lifting deviceaccording to embodiments of the present disclosure;

FIG. 10 illustrates a device member for a pipe coil lifting deviceaccording to embodiments of the present disclosure;

FIG. 11 illustrates the positions of an end-member attached todevice-member that is attached to a central member according toembodiments of the present disclosure;

FIG. 12 illustrates a cross-member for a pipe coil lifting deviceaccording to embodiments of the present disclosure;

FIG. 13 illustrates an upper surface of a pipe coil lifting deviceaccording to embodiments of the present disclosure;

FIG. 14 illustrates a cross sectional view of the upper surface of apipe coil lifting device according to embodiments of the presentdisclosure;

FIG. 15 illustrates the position of a bar at the end of the uppersurface of a pipe coil lifting device according to embodiments of thepresent disclosure;

FIG. 16 illustrates the half pipe attached to the channel for a pipecoil lifting device according to embodiments of the present disclosure;

FIG. 17 illustrates a top view of a pipe coil lifting device accordingto embodiments of the present disclosure; and

FIG. 18 illustrates a view from under a pipe coil lifting deviceaccording to embodiments of the present disclosure.

DETAILED DESCRIPTION

Embodiments of the present disclosure relate generally to a pipe coildevice for use in transporting and/or deploying coils of pipe using aforklift or a crane. Coils of pipe may be self supported, for example,using bands to hold coils together, or coils of pipe may be supportedaround a reel (which may be referred to as a reel of pipe). Coil liftingdevices according to embodiments of the present disclosure for uselifting coils held together with using bands or straps may include asuperstructure with a central longitudinal member and one or morecross-members, with two channels at the outside long-edges of thesuperstructure attached to the cross-members, with a convex uppersurface attached to the cross-members of the superstructure.

Embodiments of the present disclosure will be described below withreference to the figures. In one aspect, embodiments disclosed hereinrelate to embodiments for a Half-Moon Lifting Device configured totransport coils of flexible pipe.

FIG. 1 shows a diagram of a Half-Moon Lifting Device 24 disposed on thetines 40 of a forklift 30 according to embodiments of the presentdisclosure. A forklift may be equipped with a hydraulic side-shiftfunction for moving or adjusting the forks or tines.

Coil of pipe 102 may be formed by wrapping pipe into a coil with aninterior channel 104 formed axially therethrough, where the coil of pipe102 may be moved as a single package or bundle of coiled pipe, as shownin FIG. 2 . In FIG. 3 a coil of pipe 102 is illustrated disposed on theHalf-Moon Lifting Device 24. Each complete turn of coiled pipe may bereferred to as a wrap of pipe. Multiple wraps of pipe in a coil of pipemay be configured in columns along an axial dimension of the coil ofpipe and/or configured in layers along a radial dimension of the coil ofpipe. For example, multiple columns of wraps may be formed along anaxial direction of the coil of pipe, where the axial dimension of thecoil of pipe is based on the diameter of the pipe and the number andaxial position of wraps forming the coil of pipe 102. Further, multiplelayers of wraps may be formed along a radial direction of the coil ofpipe, where the radial dimension of the coil of pipe is based on thediameter of the pipe and the number and radial position of the wrapsforming the coil of pipe.

As shown in FIG. 2 , coil of pipe 102 may be one or more layers (e.g.,layers 108 and 110) of pipe packaged or bundled into a larger coil. Coilof pipe 102 may include at least one or more layers of pipe that hasbeen coiled into a particular shape or arrangement. As shown in FIG. 2 ,coil of pipe 102 is coiled into a substantially cylindrical shape havingsubstantially circular bases 103 and 105 formed on each end of coil ofpipe 102, where the axial dimension of coil of pipe 102 is measuredbetween the two bases 103, 105. In addition, the coil of pipe may beattached to a pipe coil skid 106.

A pipe, as understood by those of ordinary skill, may be a tube toconvey or transfer any water, gas, oil, or any type of fluid known tothose skilled in the art. The pipe used to make up coil of pipe 102 maybe made of any type of materials including without limitation plastics,metals, a combination thereof, composites (e.g., fiber reinforcedcomposites), or other materials known in the art.

In one or more embodiments, the pipe used to make up coil of pipe 102may be a flexible type of pipe. Flexible pipe is used frequently in manyapplications, including without limitation, both onshore and offshoreoil and gas applications. Flexible pipe may include Flexible CompositePipe (FCP) or Reinforced Thermoplastic Pipe (RTP). A FCP/RTP pipe mayitself be generally composed of several layers. In one or moreembodiments, a flexible pipe may include a high-density polyethylene(“HDPE”) pipe having a reinforcement layer and an HDPE outer coverlayer. Additionally, various types of polyethylene are available forflexible pipe composition. Other polymers may also be used such asnylon, PVDF, polypropylene and many others. Thus, flexible pipe mayinclude different layers that may be made of a variety of materials andalso may be treated for corrosion resistance. For example, in one ormore embodiments, pipe used to make up a coil of pipe may have acorrosion protection shield layer that is disposed over another layer ofsteel reinforcement. In this steel reinforced layer, helically woundsteel strips may be placed over a liner made of thermoplastic pipe.Flexible pipe may be designed to handle a variety of pressures. Further,flexible pipe may offer unique features and benefits versus steel/carbonsteel pipe lines in the area of corrosion resistance, flexibility,installation speed and re-usability.

Coils of pipe may be made with coil having an outer diameter ranging,for example, from about 2 inches (5.1 cm) to about 10 inches (25.4 cm).However, pipe having other dimensions may be coiled to form a coil ofpipe and loaded for transportation according to embodiments of thepresent disclosure. Accordingly, pipe that that may be spooled or coiledinto coil of pipe 102 may be made to suit a number of dimensions and mayhave any diameter useful to a particular project.

As known to those of ordinary skill in the art, pipe used to make upcoil of pipe 102 may be coiled using spoolers or other coiler machinessuited for such a function. Those of ordinary skill will recognize thatthe present disclosure is not limited to any particular form of coileror other device that may be used to form pipe into a coil. Coiling pipeinto a coil of pipe, such as 102, assists when transporting pipe, whichmay be several hundred feet in length in one or more embodiments.Further, coil of pipe 102 may be assembled as a coil to facilitatedeployment of the coil. Deployment, as described above and used herein,may refer to the action of unspooling or unwinding the pipe from coil ofpipe 102.

After being assembled into a coil, coil of pipe 102 may include aninterior channel 104 formed axially through the coil of pipe 102.Interior channel 104 is a bore disposed generally in the center of coilof pipe 102. Interior channel 104 is substantially circular shaped. Thecoil of pipe 102 may have an outer diameter (OD) and an inner diameter(ID), where the inner diameter is defined by the interior channel.

In one or more embodiments, coil of pipe 102 may have an outer diameterranging from about 60 inches (1.5 m), which may occur, for example, whencoil of pipe 102 has at least two layers of 2 inch pipe, to about 192inches (4.9 m). In one or more embodiments, a coil of pipe may have aninner diameter ranging, for example, from about 84 inches (2.1 m) toabout 126 inches (3.2 m). Further, in one or more embodiments, a coil ofpipe may have an axial dimension (width) ranging from about 5 inches(12.7 cm) to about 92 inches (2.3 m). However, these are merelyexemplary measurements. Those of ordinary skill in the art willappreciate that any range of dimensions (inner and outer diameters andwidth) may be accommodated using one or more embodiments.

Skid 106 illustrated in FIG. 2 may be a platform upon which coil of pipe102 may be disposed to hold the coil of pipe 102 in a verticalorientation. In one or more embodiments, coil of pipe 102 may be loadedand unloaded for transportation while remaining on skid 106.

Now turning to FIG. 4 various illustrative embodiments of a pipe coillifting device 24 (‘Half-Moon Lifting Device’) are illustrated. Thedevice is used to access the interior diameter of a coil of pipe 102,and used with a forklift to carry and move the pipe. Various aspects ofa lifting device 24 suitable for moving a coil of pipe with a wide rangeof diameters are illustrated in FIGS. 4 to 16 . It should be noted thatvarious sizes of lifting devices are suitable for moving a coil of pipe.

As illustrated in FIG. 5 , the device has a superstructure including apair of channels (or channel portions) 1, the channels or channelportions accommodating forks or tines of a forklift. In addition thechannels may accommodate means for securing the lifting device to thetines of a forklift. An upper surface 2 of the device 24, may be archedto attach to the top of cross-members 6, of which there may be one ormore, for example five cross-members 6 as illustrated in FIG. 5 . Theupper surface 2 may overlap onto the outside surface of the channels 1(see also FIG. 14 illustrating overlap portion to attach to channel).This overlap of surface 2 onto channels 1 as well as surface 2 attachedto cross-member 6 helps provide stiffness to the lifting device 24. Thedevice 24 may include device member 3, of which there may be two for thedevice 24, (see also FIG. 10 ). End member 4 as illustrated, of whichthere may be two on either end of the device 24, (see also FIG. 9 ) issuitable for attaching the device 24 to cables for use with a crane.Device member 3 and end member 4 are attached to a longitudinal centralmember 5. Cross-members 6, illustrated in FIG. 8 and FIG. 12 , of whichthere may be one or more, has an upper edge that meets upper surface 2.A half-pipe 7, for example a schedule 40 pipe split lengthwise, may beattached to the outer surfaces of channels 1 to provide protection tothe interior surfaces of coiled pipe so as to protect the pipe materialfrom angled surfaces on the lifting device 24 (see also FIG. 16 ).Likewise, on one or either end of upper surface 2, a round bar 8, forexample a ¼ inch or similar appropriate size, may be attached along thearcuate edge that will also provide sharp-edge protection to the pipematerial to be moved with the lifting device 24 and is furtherillustrated in FIG. 15 .

In certain illustrative embodiments, lifting device 24 can be formed ofsteel or other metal material. In certain illustrative embodiments thelifting devices are capable of a 20 ton rating, or for a rated load of40,000 lbs.

Lifting device 24 supporting various diameter coils, is illustrated incross-section with FIG. 6 . FIG. 7 illustrates another cross-sectionalview showing the spacing between the cross-members 6. FIG. 8 illustratesstill another cross-sectional view through lifting device 24 showingcross member 6, central member 5 and channels 1. FIG. 9 illustrates twoviews of end-member 4. FIG. 10 illustrates further detail of devicemember 3. FIG. 11 illustrates the positions of end-member 4 attached todevice-member 3 shown attached to central member 5. FIG. 12 illustratesa cross-member 6. FIG. 13 illustrates a side view of upper surface 2.FIG. 14 is cross section of upper surface 2.

FIG. 15 illustrates the round bar 8 may be attached along the arcuateedge and may extend at least partially over channels 1. FIG. 16illustrates half-pipe 7, which may be a schedule 40 pipe (for examplesplit in two pieces along the central axis) for attachment to thechannel 1 or the channel 1 and the overlap portion of upper surface 2.

In general a lifting device 24 is capable of use with various diametercoils without damaging the outside surfaces of the coils of pipecontacted by the long-sides of the lifting device 24 during movement bya forklift.

FIGS. 17 and 18 illustrate perspective views of another embodiment ofthe pipe coil lifting device 24, which may capable of a 34 ton rating,or for a rated load of 68,000 lbs. Elements in common with those shownin previous figures are labeled with the same reference numerals. Asshown in FIGS. 17 and 18 , the upper surface 2 comprises a first uppersurface section 9 attached to a first support 10 coupled to a first sideof the central member 5, and a second upper surface section 11 attachedto a second support 12 coupled to a second side of the central member 5.In other embodiments, the upper surface 2 may be configured as a singlesection or divided into more than two sections. The first and secondsides of the central member 5 may be opposite from one another. Incertain embodiments, the central member 5 may be configured as a singleplate of steel or similar material. In further embodiments, the liftingdevice 24 may include a plurality of first supports 10 and a pluralityof second supports 12 spaced apart from one another to provide supportalong the length of the lifting device 24. In the illustratedembodiment, the lifting device 24 includes eight (8) first supports 10and eight (8) second supports 12 with the first and eighth supports 10and 12 serving as ends of the lifting device 24.

In certain embodiments, a coating 13 may be applied to an outer surfaceof the upper surface 2. For example, the coating 13 may be a rubberizedcoating, polymer, paint, sheet, or layer applied to the upper surface 2and configured to provide a surface that will reduce the likelihood ofdamaging the interior channel 104 of the coil of pipe 102. In furtherembodiments, the upper surface 2 may include curves 14 near the channels1 to also reduce the likelihood of damaging the coil of pipe 102. Insuch embodiments, the half-pipes 7 may be omitted.

In the illustrated embodiment of FIGS. 17 and 18 , the central member 5includes end members 4 suitable for attaching the lifting device 24 tocables for use with a crane. The end members 4 may be attached to thecentral member 5 or may be an integral or formed part of the centralmember 5. Further, the channels 1 may be attached to lower surfaces 15of the supports 10 and 12.

In one nonlimiting embodiment a pipe coil lifting device comprises asuperstructure with a central longitudinal member and at least onecross-member, with two channels at the outside long-edges of thesuperstructure attached to the cross-members, and a convex upper surfaceis attached to the cross-members of the superstructure.

In other aspects, the pipe coil lifting device upper surface is attachedto and overlaps upon the outside surface of the channels. Further, thepipe coil lifting device may have a device member attached to thecentral member and an end member attached to the central member. Thecentral longitudinal member may extend beyond the upper surface andcomprise an end member suitable for attaching cable fittings. Half-pipesmay be attached to the lateral outside surfaces of the channels. A ¼inch round bar may be attached to the arcuate edge of the convex uppersurface.

In another nonlimiting embodiment, a pipe coil lifting device comprisesa superstructure having a pair of longitudinal channel portions attachedto and forming sides of an outermost portion of the superstructure. Alongitudinal central member extends between the channel portions withthe superstructure having at least one cross-member perpendicular andattached to the central member and channel portions. The top of thesuperstructure forms a convex upper surface attached to the channelportions, central member and cross-members.

In other aspects, the upper surface of the pipe coil lifting deviceoverlaps upon the outside surface of the channel portions. A devicemember may be attached to the central member and an end member attachedto the central member. The longitudinal central member may extend beyondthe upper surface and comprise an end member for attaching cablefittings. Half-pipes may be attached to the lateral outside surfaces ofthe channel portions. A ¼ inch round bar attached to the arcuate edge ofthe convex upper surface.

In still another nonlimiting embodiment, a method is provided forlifting a pipe coil the method comprising lifting a coil of flexiblepipe with a pipe coil lifting device disposed on a forklift, the pipecoil lifting device comprising a superstructure with a central memberand at least one cross-member perpendicular to the cross member, withtwo channels at the outside long-edges of the superstructure, with aconvex surface attached to the cross-members of the superstructure. Whenusing the lifting device with a forklift, a hydraulic side-shiftfunction of the forklift may be used to secure the lifting device to thetines of the forklift. Alternatively, the lifting device may be attachedfor moving a pipe coil with a crane using a cable or chain attached toend members of the central member of the device.

While the present disclosure has been described with respect to alimited number of embodiments, for example lifting devices for coilswith inside diameters of seven and nine feet, those skilled in the art,having benefit of this disclosure, will appreciate that otherembodiments may be devised which do not depart from the scope of thedisclosure as described herein. Accordingly, the scope of the disclosureshould be limited only by the attached claims.

What is claimed is:
 1. A pipe coil lifting device comprising: a firstchannel configured to accommodate a first tine of a forklift; a secondchannel configured to accommodate a second tine of the forklift; aplurality of cross-members that each extends non-parallel relative tothe first channel and the second channel; a central longitudinal memberthat extends non-parallel relative to the plurality of cross-members;and a convex upper plate attached to the central longitudinal member andeach of the plurality of cross-members.
 2. The pipe coil lifting deviceof claim 1, wherein the central longitudinal member extendsperpendicularly through one or more of the plurality of cross-members.3. The pipe coil lifting device of claim 1, wherein: the convex upperplate is attached on top of the central longitudinal member, on top ofeach of the plurality of cross-members, to a first outer side surface ofthe first channel, and to a second outer side surface of the secondchannel; and each cross-member in the plurality of cross-members isattached to a first top surface and a first inner side surface of thefirst channel and to a second top surface and a second inner sidesurface of the second channel.
 4. The pipe coil lifting device of claim1, wherein each cross-member in the plurality of cross-memberscomprises: a first support member attached to a first top surface of thefirst channel and a first side surface of the central longitudinalmember; and a second support member attached to a second top surface ofthe second channel and a second side surface of the central longitudinalmember.
 5. The pipe coil lifting device of claim 1, wherein the convexupper plate comprises: a first upper plate section attached on top ofeach of the plurality of cross-members and to a first side surface ofthe central longitudinal member; and a second upper plate sectionattached on top of each of the plurality of cross-members and to asecond side surface of the central longitudinal member.
 6. The pipe coillifting device of claim 1, wherein: the convex upper plate is metal; andan upper surface of the convex upper plate is coated with rubber,polymer, or paint.
 7. The pipe coil lifting device of claim 1,comprising: a first device member secured to a first end of the centrallongitudinal member; a first end member secured to the first devicemember, wherein the first end member comprises a first cable openingconfigured to enable a cable to be attached to the pipe coil liftingdevice; a second device member secured to a second end of the centrallongitudinal member opposite the first device member; and a second endmember secured to the second device member, wherein the second endmember comprises a second cable opening configured to enable the cableto be attached to the pipe coil lifting device.
 8. A pipe coil liftingdevice comprising: a plurality of forklift channels; a cross-member thatextends non-parallel relative to the plurality of forklift channels; acentral longitudinal member that extends non-parallel relative to thecross-member; a first end member secured to a first end of the centrallongitudinal member, wherein the first end member comprises a firstcable opening configured to enable a crane cable to be attached to thepipe coil lifting device; a second end member secured to a second end ofthe central longitudinal member opposite the first end member, whereinthe second end member comprises a second cable opening configured toenable the crane cable to be attached to the pipe coil lifting device;and a convex upper plate attached to the cross-member and the centrallongitudinal member.
 9. The pipe coil lifting device of claim 8,comprising: a first device member secured between the first end of thecentral longitudinal member and the first end member; and a seconddevice member secured between the second end of the central longitudinalmember and the second end member.
 10. The pipe coil lifting device ofclaim 8, wherein the central longitudinal member extends perpendicularlythrough the cross-member.
 11. The pipe coil lifting device of claim 8,wherein: the plurality of forklift channels comprises: a first forkliftchannel configured to accommodate a first tine of a forklift; and asecond forklift channel configured to accommodate a second tine of theforklift; the convex upper plate is attached to a first outer sidesurface of the first forklift channel and a second outer side surface ofthe second forklift channel; and the cross-member is attached to a firsttop surface and a first inner side surface of the first forklift channeland to a second top surface and a second inner side surface of thesecond forklift channel.
 12. The pipe coil lifting device of claim 8,wherein the plurality of forklift channels comprises: a first forkliftchannel configured to accommodate a first tine of a forklift; and asecond forklift channel configured to accommodate a second tine of theforklift; and the cross-member comprises: a first support memberattached to a first top surface of the first forklift channel and afirst side surface of the central longitudinal member; and a secondsupport member attached to a second top surface of the second forkliftchannel and a second side surface of the central longitudinal member.13. The pipe coil lifting device of claim 8, wherein the convex upperplate comprises: a first upper plate section attached on top of thecross-member and to a first side surface of the central longitudinalmember; and a second upper plate section attached on top of thecross-member and to a second side surface of the central longitudinalmember.
 14. The pipe coil lifting device of claim 8, wherein: the convexupper plate is metal; and an upper surface of the convex upper plate iscoated with rubber, polymer, or paint.
 15. A pipe coil lifting devicecomprising: a first channel configured to accommodate a first tine of aforklift; a second channel configured to accommodate a second tine ofthe forklift; a cross-member that extends perpendicular to the firstchannel and the second channel; a central longitudinal member thatextends perpendicular relative to the cross-member and parallel relativeto the first channel and the second channel; and a convex upper plateattached to the cross-member and the central longitudinal member,wherein: the convex upper plate is metal; and an upper surface of theconvex upper plate is coated with rubber, polymer, or paint.
 16. Thepipe coil lifting device of claim 15, wherein the central longitudinalmember extends through the cross-member.
 17. The pipe coil liftingdevice of claim 15, comprising: a first device member secured to a firstend of the central longitudinal member; a first end member secured tothe first device member, wherein the first end member comprises a firstcable opening configured to enable a cable to be attached to the pipecoil lifting device; a second device member secured to a second end ofthe central longitudinal member opposite the first device member; and asecond end member secured to the second device member, wherein thesecond end member comprises a second cable opening configured to enablethe cable to be attached to the pipe coil lifting device.
 18. The pipecoil lifting device of claim 15, wherein: the cross-member is attachedto a first top surface and a first inner side surface of the firstchannel and to a second top surface and a second inner side surface ofthe second channel; and the convex upper plate is attached to a firstouter side surface of the first channel and a second outer side surfaceof the second channel.
 19. The pipe coil lifting device of claim 15,wherein the cross-member comprises: a first support member attached to afirst top surface of the first channel and a first side surface of thecentral longitudinal member; and a second support member attached to asecond top surface of the second channel and a second side surface ofthe central longitudinal member.
 20. The pipe coil lifting device ofclaim 15, wherein the convex upper plate comprises: a first upper platesection attached on top of the cross-member and to a first side surfaceof the central longitudinal member; and a second upper plate sectionattached on top of the cross-member and to a second side surface of thecentral longitudinal member.